Methods of rolling seamless tubes are known using a continuous multiple stand rolling mill and a restrained mandrel which is advanced at a constant controlled speed during rolling.
For rolling using an installation of this type, an ingot or hollow bloom is set up on a support in front of the multiple stand rolling mill, and a mandrel is inserted through the said bloom as far as the rolling position. In this position, the head of the mandrel rests, behind the bloom, against a restraining cross-piece which is initially stationary. Upon the introduction of the bloom between the first pair of rollers of the mill, the forward movement of the cross-piece restraining the mandrel is commenced, preferably by withdrawing oil through a constant delivery pump from oleodynamic actuator cylinders controlling the movement of the cross-piece. In this way the speed of advance of the mandrel is controlled at a predetermined rate which depends upon the degree of permissible heating of the surface layer of the said mandrel. The mandrel is lubricated during rolling by the injection of lubricant on to the internal surface of the bloom through a conduit located within the mandrel, the latter being constantly cooled on the inside by means of a continuous current of cold water under pressure (see for example French Patent No. 1,458,432).
After rolling of the tube and extraction of the mandrel, the latter, secured to a longitudinal conveyor device, is withdrawn through the rolling mill and carried to its initial position for the start of a fresh cycle. In order to cool the mandrel completely it is thrust laterally, in the outer cooling position, under coolant showers, whilst a second mandrel, which was in the position of rest and cooling under the showers during the working cycle of the first mandrel, is automatically placed in the working position by the displacement of the first mandrel.
By means of this alternation in working of the two mandrels, a fairly satisfactory production rate can be achieved, but this method has various disadvantages:
THE IDLE PERIODS OF INSERTION, EXTRACTION, RETURN AND LATERAL DISPLACEMENT OF THE MANDREL REDUCED BY AT LEAST 50% THE PRODUCTIVE TIME OF THE WHOLE CYCLE;
THE CHANGING OF THE MANDRELS, SECURED TO MANDREL CONTROL DEVICES, ENTAILS LOSS OF TIME AND INTERRUPTS THE TUBE PRODUCTION;
THE CENTRE DISTANCE BETWEEN TWO SUCCESSIVE STRANDS OF THE ROLLING MILL, WHICH IS A DETERMINING FACTOR FOR THE SPEED OF MOVEMENT OF THE MANDREL, HAS TO BE SO LIMITED AS NOT TO INCREASE UNDULY THE INSERTION AND RETURN STROKES OF THE MANDRELS, AS WELL AS THEIR WEIGHT.
An object of this invention is to eliminate as far as possible the disadvantages referred to above, by providing a manufacturing method by means of which idle times can be noticeably reduced, and rational operational inter-linking achieved, especially with regard to the displacement and cooling of the mandrels.